run_target: do not yield broken names with subdirs

A run_target object created in a subdir/meson.build always has a ninja
rule name of "name", not "subdir/name".

Fixes #9175

mesonbuild/backend/backends.py

@@ -358,7 +358,10 @@ class Backend:
 
     @lru_cache(maxsize=None)
     def get_target_dir(self, target: T.Union[build.Target, build.CustomTargetIndex]) -> str:
-        if self.environment.coredata.get_option(OptionKey('layout')) == 'mirror':
+        if isinstance(target, build.RunTarget):
+            # this produces no output, only a dummy top-level name
+            dirname = ''
+        elif self.environment.coredata.get_option(OptionKey('layout')) == 'mirror':
             dirname = target.get_subdir()
         else:
             dirname = 'meson-out'

test cases/unit/65 alias target/meson.build

@@ -13,3 +13,5 @@ custom_target = custom_target('custom-target',
 )
 
 alias_target('build-all', [exe_target, custom_target])
+
+subdir('subdir')

test cases/unit/65 alias target/subdir/meson.build

@@ -0,0 +1,6 @@
+r = run_target('run-target',
+  command: [python3, '-c', 'print("a run target was here")']
+)
+
+alias_target('aliased-run', r)
+

unittests/allplatformstests.py

@@ -3076,6 +3076,8 @@ class AllPlatformTests(BasePlatformTests):
         self.run_target('build-all')
         self.assertPathExists(os.path.join(self.builddir, 'prog' + exe_suffix))
         self.assertPathExists(os.path.join(self.builddir, 'hello.txt'))
+        out = self.run_target('aliased-run')
+        self.assertIn('a run target was here', out)
 
     def test_configure(self):
         testdir = os.path.join(self.common_test_dir, '2 cpp')